1. Field of the Invention
This invention relates generally to a method for generating steam by recovering heat from the exhaust gas of a gas turbine operating in a combined cycle power plant. The steam is expanded in one or more steam turbines which, together with the gas turbine, produce electrical power.
2. Description of the Prior Art
The low capital cost, short lead times and flexibility of gas turbine-based power plants make them particularly attractive to electrical utilities as a means for generating electrical power. Unfortunately, the inefficiency of a gas turbine standing alone, referred to as a simple cycle system, is relatively low compared to conventional fired boil steam turbine systems. The major source of this inefficiency is inherent in the Brayton cycle on which the gas turbine operates. The Brayton cycle operates in three phases--first, work is performed on the fluid (air in the case of a gas turbine) by isentropic compression in a compressor; second, heat is added to the fluid isobarically in a combustor; and, third, the hot compressed fluid is isentropically expanded back down to its initial pressure in the turbine. During the expansion phase much of the energy imparted to the fluid as a result of the compression and heating is recovered in the form of useful work. However, a significant portion of the energy remains in a relatively high-temperature, low-pressure form which, as a practical matter, cannot be recovered by further expansion in the turbine. In a simple cycle system this energy is lost to the atmosphere when the gas exhausting from the gas turbine is vented to atmosphere. The magnitude of this energy loss can be appreciated by noting that in a typical simple cycle system, air inducted into the compressor at ambient temperature is heated to approximately 2000.degree. F. in the combustor prior to expansion in the turbine but is only cooled to approximately 1000.degree. F. when vented to atmosphere after expansion in the turbine. Thus, the portion of the fuel burned in the combustor which was used to raise the temperature of the ambient air to 1000.degree. F. is wasted, resulting in poor overall thermodynamic efficiency.
Consequently, substantial effort has been expended in developing methods for recovering the energy available in the gas exhausting from a gas turbine. One of the most successful methods involves the transfer of latent heat from the hot exhaust gas to pressurized feedwater in a heat recovery steam generator (hereinafter HRSG). The HRSG generates steam which is expanded in a steam turbine producing additional rotating shaft power. Since steam turbines operate on the Rankine cycle, rather than the Brayton cycle, power plants employing such a heat recovery method are termed combined cycle power plants.
Typically, a HRSG is comprised of a large duct through which the exhaust gas flows. The duct encloses banks of tubes through which the water/steam flows and over which the gas turbine exhaust gas flows. The surfaces of the tubes provide heat transfer surfaces There are three basic components in which heat is transferred in a typical HRSG, each comprised of a bundle of tubes: an economizer in which the feedwater is heated to near-saturation temperature; an evaporator in which the water heated in the economizer is converted to steam; and a superheater in which the temperature of the saturated steam from the evaporator is raised into the superheat region.
In order to obtain maximum efficiency of the steam turbine, it is desirable to generate steam at a high temperature and pressure. However, unless supplemental fuel is burned in the exhaust gas, an inefficient practice, the steam temperature is limited to the temperature of the exhaust gas entering the HRSG. The maximum pressure of the steam is also limited by the temperature of the exhaust gas since the saturation temperature of steam increases with its pressure and only the portion of the heat in the exhaust gas which is above the saturation temperature of the water in the evaporator can be used to generate steam. Hence, although increasing steam pressure increases steam turbine efficiency, it also reduces the quantity of the steam generated. Thus maximum heat recovery, and therefore maximum plant power output, are obtained by optimizing the relationship between the steam pressure and steam flow.
One optimization method utilizes a HRSG which generates steam at multiple pressure levels by employing a separate evaporator at each pressure level. The gas turbine exhaust gas is directed to the highest pressure evaporator first, then each successive lower pressure level evaporator. Thus, although the temperature of the gas entering the evaporator decreases at each successive pressure level, the saturation pressure and hence saturation temperature of the water in each successive evaporator is also reduced, so that additional steam may be produced at each pressure level.
Thus, it is desirable to devise a method of heat recovery which employs the optimum number of pressure levels, each operating at its optimum pressure, and which utilizes the steam produced at each pressure level in the optimum manner.
In many earlier combined cycle power plants, feedwater returned from the condenser at low temperature was not entered directly into the HRSG for heating prior to deaeration. Instead, feedwater heating was accomplished indirectly, using steam generated in a low-pressure evaporator or extracted from an intermediate stage of a low-pressure steam turbine. Although such methods ensured that the exhaust gas would not be cooled below its acid dew point temperature, they limited the amount of heat which could be recovered from the exhaust gas and reduced the steam available to generate electrical power.
Thus, it would be desirable to devise a method of heating the feedwater using heat removed from the exhaust gas directly, and to do so without encountering the dangers of acid corrosion due to excessive cooling of the exhaust gas in the HRSG.